KIN 140 Section 2 Dr Mike Walsh
Life comes from other life and disease comes from germs (and a few other things)
I. Infectious Diseases
Adisease is any deviation from normal functioning.An infectious disease is a disease in
which a pathogen is present and may multiply.
Of course, this does not include, for example, mercury poisoning.
Apathogen is an agent that produces disease. Types of pathogens include the following:
These are single-celled organisms with their own DNAand no membrane bound
organelles. About 1500 are well classified and about 100 of these are pathogens. Some
bacteria are very helpful to aid us in such things as digestion of food in our intestines
(remember the intestines are outside of us). The inside of our body is virtually sterile.
Bacteria within our body are almost always pathogenic and attacked by our immune
system. Those bacteria that survive the body’s defenses multiply and cause infection. The
infection can occur in virtually any tissue in our body.
Note: the gastrointestinal tract is not in our body
Lets keep bacteria in some perspective. In terms of mass, there are more bacteria on this
planet than plants and animals combined)
Examples of Bacterial Infection
Tuberculosis. The number one infectious disease in the world
Meningitis (also other causes)
Tetanus. Not infectious.
2. Viruses 2
These are the smallest ‘living’pathogens. They consist primarily of small amounts of
genetic material (DNAor RNA) covered by a protein coat. They are a borderline life
form because they require other cells to reproduce. Unlike bacteria, they do not have their
own life cycle. Viruses can reproduce in a variety of human cell types (and other cell
types such as bacteria or plants).
Examples of Viral Infection
Flu. The common cold is caused by the influenza virus in its various forms.
Polio. The poliovirus reproduces in motor neurons which cannot be replaced and thus the
consequence of such an infection is severe.
Herpes. The herpes virus can be in one’s body for life. When conditions are right, the
formally dormant virus begins to reproduce (an opportunistic virus).
Warts human papilloma virus
These are primitive plant forms that may be unicellular (yeast) or multicellular (molds).
Fortunately fungal infections are restricted to the skin and only about 50 of the thousands
of known fungi cause disease in humans.
Examples of Fungal Infection
Candida albicans (candidiasis). This is a yeast infection in a woman’s vagina. This
normal resident can cause problems when it suddenly multiplies.
These are single-celled animals. Many of these diseases are recurrent as the pathogen
alternates between inactive and active phases.
Examples of Protozoa Infection
Giardia (Beaver fever). This can arise for drinking untreated water resulting in diarrhea,
nausea, and abdominal cramps.
5. Parasitic Worms.
These include tapeworms, ringworms, and leeches. They generally arise from
contaminated food and drink.
These are protein particles only. Prions are proteins found in mammalian cell walls. They
occur more frequently in the central nervous system. We do not know their normal
function. But aberrant prions can cause mad cow disease (bovine spongiform
B. Chain of Infection
The pathogen can be any disease-causing agent, infectious or otherwise and include those
The reservoir can be human, animal, inanimate, or environmental. In the reservoir, the
pathogen can either be stable or multiply. If it multiplies in a living host, symptoms of the
disease will develop.
3. Portal of Exit
The pathogen can leave a human host by many ‘portals’including all bodily fluids. Saliva
for mumps, mucous membranes for sexually transmitted diseases (STDs), blood for HIV,
feces for intestinal infections, and throat and nose mucous for colds and flu.
4. Mode of Transmission
Acommunicable disease is one that can be transmitted from one host to another (a
contagious disease is one that is spread easily). The pathogen must travel from the
reservoir to a susceptible host. This can be done by 2 classification mechanisms.
a. Direct Transmission 4
Human to human transmission can be done via
body surface contact (kissing, shaking hands)
Droplet spread. Inhalation of someone’s breath (especially a sneeze)
Fecal-oral spread. Feces on the hands brought into contact with the mouth.
b. Indirect T